Over the years researchers have attempted to establish the human filariid, Wucheria bancrofti, in a variety of laboratory animals without success. This mosquito-transmitted lymphatic-dwelling parasite affects approximately 100 million people in tropical and subtropical regions of the world, and in the chronic stage it may lead to the clinical manifestation known as elephantiasis. In past reports we summarized our success in establishing an infection in 6 of 11 patas monkeys inoculated with this parasite. The last animal in this series was inoculated with 150 infective larvae in October 1992 with a low microfilaremia first detected seven months later. This has been the usual prepatent period for all but one of the six infected animals. Ten months later the microfilaremia reached a peak of nearly 100 mf/20 ul of blood, and it still continues at a low infection level (3-10 rnf/20 ul) to this date, i.e. for nearly four years. However, efforts to infect the natural mosquito vector we maintain in colony, Culex quinquefasciatus, by blood feeding directly on this animal have been largely unsuccessful. Studies now show that the difficulty rests with the mosquito which has been maintained in colony for nearly two years, since we no longer can infect them even with cryopreserved W. bancrofti-infected blood from Haitian patients. This never had been a problem in past years. This research amply demonstrates that the patas monkey can serve as a suitable laboratory host for W. bancrofti considering 1-2 microfilaremic animals have always been on hand for the past twelve years.